Question: Compute $\cos 72^\circ.$
Solution: Let $a = \cos 36^\circ$ and $b = \cos 72^\circ.$  Then by the double angle formula,
\[b = 2a^2 - 1.\]Also, $\cos (2 \cdot 72^\circ) = \cos 144^\circ = -\cos 36^\circ,$ so
\[-a = 2b^2 - 1.\]Subtracting these equations, we get
\[a + b = 2a^2 - 2b^2 = 2(a - b)(a + b).\]Since $a$ and $b$ are positive, $a + b$ is nonzero.  Hence, we can divide both sides by $2(a + b),$ to get
\[a - b = \frac{1}{2}.\]Then $a = b + \frac{1}{2}.$  Substituting into $-a = 2b^2 - 1,$ we get
\[-b - \frac{1}{2} = 2b^2 - 1.\]Then $-2b - 1 = 4b^2 - 2,$ or $4b^2 + 2b - 1 = 0.$  By the quadratic formula,
\[b = \frac{-1 \pm \sqrt{5}}{4}.\]Since $b = \cos 72^\circ$ is positive, $b = \boxed{\frac{-1 + \sqrt{5}}{4}}.$